1. Field of the Invention
The present invention relates to an automatic focusing apparatus that uses different automatic focusing methods and that performs lens drive control for moving a focus lens to the in-focus point, and an image pickup apparatus such as a video camera.
2. Description of the Related Art
In recent years, automatic focusing apparatuses for video cameras have mainly used a method for focusing in which the sharpness (contrast) of an image is detected from the output signal from an image pickup device, the sharpness is referred to as focus evaluation value (focus signal), and the focus lens position is controlled so that the value is maximized. This method is called hill-climbing AF method (hereinafter referred to as contrast AF method).
In general, the level of the high-frequency component of the output signal of the image pickup device extracted by a band-pass filter of a certain band is used as the focus evaluation value in the contrast AF. The reason is that when a normal object image is photographed, as shown in FIG. 7, the focus evaluation value increases with focusing, and the point at which its level is highest is the in-focus point.
AF methods also include an internal ranging phase difference AF method, which is commonly used in silver-halide film single-lens reflex cameras. In this method, a light beam passing through the exit pupil of a photographing lens is split into two light beams, and the light beams are received by a pair of ranging sensors, respectively. The amount of deviation of the signal output according to the amount of received light, that is to say, the amount of relative positional deviation in the beam splitting direction is detected. Thereby, the amount of deviation in the focusing direction of the photographing lens is directly obtained. Therefore, if accumulating operation is performed by the ranging sensors once, the amount and direction of focusing deviation can be obtained, and high-speed focusing operation is possible.
There is another type of phase difference AF method, in which ranging sensors are provided independently from a photographing lens. This method is called external ranging phase difference AF method. In the external ranging phase difference AF method, a light beam received from an object is split into two light beams, and the light beams are received by a pair of ranging sensors, respectively. The amount of deviation of the signal output according to the amount of received light, that is to say, the amount of relative positional deviation in the beam splitting direction is detected. Thereby, the object distance is obtained by triangulation. AF methods using external ranging sensors also include a method in which the propagation velocity is measured using an ultrasonic sensor, and a method commonly used in compact cameras and in which triangulation is performed using infrared sensors. In these methods, information corresponding to the object distance is detected.
There are proposed AF apparatuses such that a focus lens is moved close to the in-focus point by the internal ranging phase difference AF method, for example, and is thereafter driven to the in-focus point by the contrast AF method (see, for example, Japanese Patent Laid-Open No. 5-64056).
In the above known apparatuses, a focus lens is first moved to an in-focus point determined by an AF method other than the contrast AF method. Only when the focus evaluation value is larger than a predetermined amount, the apparatuses change their AF method to the contrast AF method. Drive control to an in-focus point (lens drive control) is then performed by the contrast AF method. The reason of such configuration is that the contrast AF method enables more accurate focusing.
Therefore, the in-focus point according to the contrast AF method usually differs from the in-focus points according to other AF methods in which lens drive control is performed based on information corresponding the object distance, for example, the phase difference AF method. In addition, the image pickup region where focus detection is performed by the contrast AF method does not necessarily correspond to the region on the image pickup screen subject to detection of the phase difference AF method. Therefore, in-focus points for different objects can be calculated in the contrast AF method and the phase difference AF method, respectively.
In the case where the known apparatuses have different in-focus points due to use of a plurality of AF methods, they have the following drawbacks. Although the focus lens has reached the in-focus point according to the contrast AF method, which enables accurate focusing, the focus lens is often carelessly moved to the in-focus point obtained by the phase difference AF method, for example, and thereby a defocus occurs. In addition, a phenomenon called hunting can occur, in which the focus lens reciprocates between the in-focus points obtained in the respective methods. These do not matter in the case of AF apparatuses for a still image, in which recording is not performed during AF operation. However, in the case of AF apparatuses for a moving image, these are problematic because an unnatural motion is recorded.
The focus evaluation value fluctuates significantly and is unstable during a panning operation or when the panning is completed. It is highly likely that the direction of focusing performed by the contrast AF method is wrong, and the in-focus point detected by the phase difference AF method often differs from the in-focus point detected by the contrast AF method. Therefore, the focusing when the panning is completed can take a long time.